A communication system can be seen as a facility that enables communication sessions between two or more nodes such as fixed or mobile communication devices, base stations, servers and so on. A communication system and compatible communicating nodes typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standards, specifications and related protocols can define the manner how various aspects of communication shall be implemented between communicating devices. A communication can be carried on wired or wireless carriers.
In a wireless communication system at least a part of communications between the nodes occurs over one of more wireless links. Examples of wireless systems include public land mobile networks (PLMN) such as cellular networks, satellite based communication systems and different wireless local systems, for example wireless local area networks (WLAN) and base stations providing local service areas. A wireless system can be divided into cells or other radio coverage or service areas. A radio service area is provided by a base station. Radio service areas can overlap, and thus a communication device in an area can typically send signals to and receive signals from more than one base station or radio node. An example of wireless communication systems is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP).
A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE) or terminal. A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. Typically a communication device is used for enabling receiving and transmission of communications such as speech and data. In wireless systems a communication device provides a transceiver station that can communicate with another communication device such as e.g. a base station and/or another user equipment. The communication device may access a carrier provided by a base station and transmit and/or receive communications on the carrier.
A communication system can comprise different types of radio service areas providing transmission/reception points for the users. Network nodes can also be small or local radio service area network nodes. Such nodes are called, for example, home node Bs (HNB) or femto nodes and pico NodeBs (pico-NB). Just to give an example, the range of a picocell is 200 meters or less, and a femtocell is on the order of 10 meters. The smaller radio service areas can be located wholly or partially within a larger radio service area.
A user equipment may thus be located within, and thus communicate with, more than one radio service area. The nodes of the smaller radio service areas may be configured to support local offload. The local radio nodes can also, for example, be configured to extend the range of a cell.
Typically a number of home NodeBs (HNBs) are connected or coupled to one single Home NodeB Gateway (HNB-GW), the number of which can be very high (of the order of thousands), for example in typical enterprise scenarios.
The HNB together with the HNB-GW provides the same set of functions as provided by the radio network controller (RNC) together with the node B (NB). The main difference being the division of functions between the HNB/HNB-GW and NB/RNC.
Although the HNB/HNB-GW appears to the Core Network like an RNC/NB, it does not exert strict control of the radio interface using the node B application protocol (NBAP) via the Iub interface. The HNB/HNB-GW combination applies a looser control of the HNB operation via a HNB application protocol (HNBAP) via the Iuh interface. HNBAP provides a set of functions to register HNBs and in turn individual UEs to allow integration into the existing UTRAN architecture. However, the individual HNBs to a large degree operate independently from each other, resulting in specific problems. For example, there are problems when the UE initiates a Cell Update procedure.
When on common radio resources, the relevant UE context is identified by the UTRAN-Radio Network Temporary Identifier (U-RNTI) field sent by the UE to the target HNB via the RRC Cell Update message. A U-RNTI is assigned to the UE by the HNB during the RRC connection establishment procedure so that each UE having an RRC connection in that particular HNB gets a different U-RNTI value. In order to retrieve the relevant UE context during relocation from one HNB to another, it is required that the U-RNTI is also unique within all HNBs controlled by a particular HNB-GW.
3GPP UMTS specifications allow a UE to enter the “CELL_FACH”, “CELL_PCH” or “URA_PCH” states under specific conditions. One of those conditions is a UE that once was in CELL_DCH (dedicated physical channel state) and had dedicated resources assigned, but released those dedicated Uu resources as no (or in case of CELL_FACH: only minor) data exchange took place. Leaving CELL_DCH allows for reducing power consumption and therefore battery drain. The UE context is kept within the network after the state change, in other words the UTRAN-Radio Network Temporary Identifier (U-RNTI) value that was assigned when establishing an RRC connection is still kept assigned to the UE.
Once the UE decides to go back to CELL_DCH state, it uses the still assigned U-RNTI in the RRC Cell Update message. The receiving RNC then can use this value to ask the assigning (in other words still controlling) node to relocate the UE-context to the receiving node. As far as RNCs concerned, the receiving node RNC can easily deduce from the U-RNTI value the RNC originally assigning the U-RNTI value. However, where the U-RNTI was assigned by an HNB, the node receiving the Cell Update, can only address the HNB-GW but not the HNB still holding the UE context. The reason being that in HNB architecture there is no central unit which ensures uniqueness of U-RNTI values assigned by HNBs served by the same HNB-GW and furthermore that the HNB-GW has no knowledge about which HNB assigns specific U-RNTI values.
Approaches to ensure the assignment of a unique U-RNTI whenever an UE starts establishing a new RRC connection are to use a value selected at random by the HNB and modify it afterwards when the non-uniqueness has been detected by the HNB-GW, or to use the next available value out of a range of U-RNTI values assigned statically by the HNB-GW for exclusive use of the HNB, or the UE informs the source HNB about its intent to reselect to a defined target HNB and the source HNB in turn informs the target HNB accordingly. However these are problematic even the first two methods do not require modified behaviour of the UE, whereas the third method requires changes to UE behaviour. Furthermore the first method adds additional delay due to the modification of the U-RNTI and the second method lacks of flexibility.